It is known that organic light-emitting diodes (OLEDs) are made by depositing organic semiconductor materials between two electrodes on a substrate. When powered by an electric current via the electrodes, OLEDs can be used as a light source with an extended lighting surface. Nevertheless, lighting devices made of OLEDs are generally limited in sizes and light output. It is thus desirable to group together a number of OLED lighting units into an assembly so as to increase the light emitting surface. The present invention provides a method and device for grouping two or more OLED lighting units for extending the lighting surface.